The recent development of the microstructured optical fiber, in which a high index core region is surrounded by cladding having a mix of silica and air, offers new fiber properties by virtue of the large refractive-index contrast that exists between glass and air. Appropriate design of microstructured optical fiber can realize flat dispersion characteristics. For example, as described in Designing the properties of dispersion-flattened photonic crystal fibers, Ferrando et al., OPTICS EXPRESS pp. 687-697 (2001), it is possible to realize dispersion characteristic D(λ) satisfying:                               max                      λ            ∈            Ω                          ⁢                  [                      D            ⁡                          (              λ              )                                ]                    -                        min                      λ            ∈            Ω                          ⁢                  [                      D            ⁡                          (              λ              )                                ]                      ≤          Δ      ⁢                           ⁢      D        ,            and      ⁢                           ⁢                        min                      λ            ∈            Ω                          ⁢                  [                      D            ⁡                          (              λ              )                                ]                      <          D      0        <                  max                  λ          ∈          Ω                    ⁢              [                  D          ⁡                      (            λ            )                          ]              ,in a wavelength range Ω={λ|λ|≦λ2} where λ1≡1.3 [μm], λ2≡1.8 [μm], λ2−λ1=523 [nm], ΔD=2 [ps/nm/km], and D0=0 [ps/nm/km]. Such a characteristic is obtained in a structure where the radius and the pitch of the air hole are 0.316 μm and 2.62 μm, respectively.
An optical fiber having several air holes running along the fiber length, known as a “holey fiber”, or “photonic crystal fiber” is described in a paper entitled Photonic Crystal Fibres: An Endless Variety, T. A. Birks et al., IEICE Trans. Electron., V. E84-C, pp.585-592 (2001). Such fiber is also disclosed in International PCT application WO 02/39161. Dispersion tuning holes are arranged laterally displaced from the geometrical axis of optical fiber, by a distance of at least half the core radius. Provision of such additional dispersion tuning holes can be used to tune the fiber dispersion independently from the other modal properties such as the mode shape, the mode field diameter, and the effective core area. In one embodiment, the dispersion tuning holes have a cross-sectional width of less than approximately one-tenth or one-sixth of the predetermined wavelength.
An optical fiber having a flat dispersion and a small effective core area is desirable for applications such as supercontinuum light generation, optical pulse reshaping, and optical parametric amplification. In optical communication, attenuation, distortion, and timing jitter are imposed on optical signal pulses during their travel over the optical fibers and components. Supercontinuum light generation is a nonlinear optical phenomenon in which an optical pulse with relatively high power and relatively short duration is launched into a nonlinear medium and the spectrum of the pulse is broadened because of nonlinear optical phenomena and chromatic dispersion, as described in Dispersion-flattened fiber for efficient supercontinuum generation, S. Taccheo and P. Vavassori, OFC 2002, ThY5 (2002). Optical parametric amplification is an optical amplification that is caused by higher order (usually third) nonlinear susceptibility and occurs when a phase matching condition is satisfied between the lightwaves participating in the process, as described in Broadband fiber optical parametric amplifiers, M. E. Marhic et al., Opt. Lett. V. 21, pp. 573-575 (1996). In optical communication, attenuation, distortion, and timing jitter are imposed on an optical signal pulse during their travel over the optical fibers and components. Operations to remove the influences of the attenuation, distortion, and timing jitter are respectively called regeneration, reshaping, and retiming, as described in Analysis of Optical Regeneration Utilizing Self-Phase Modulation in a Highly Nonlinear Fiber, M. Matsumoto, Photon. Tech. Lett. V. 14, pp. 319-321 (2002).
The need thus exists in the prior art for the simultaneous realization of a flat dispersion and a small effective core area in microstructured fibers. Although a flat dispersion is disclosed in the above referenced Ferrando paper, the effective core area is calculated to be 36 μm2 in the structure having a hole radius of 0.316 μm and a pitch of 2.62 μm. Although the referenced prior art discloses a way to adjust chromatic dispersion independently from effective core area, it does not disclose the capability to realize a flat dispersion and a small effective core area.
In the referenced prior art structures, it is difficult to realize a low transmission loss, a high durability against UV light, and a high yield in the fiber-drawing process. In order to draw a microstructured fiber with stable quality, it is necessary to keep the furnace temperature low, or equivalently the drawing tension high, because the influence of surface tension, which causes the air holes to shrink, decreases with the decrease in temperature. However, drawing a fiber with a high tension often results in fiber breaking during drawing or structural defects in the glass of the drawn fiber. Instances of fiber breaking lower the yield of production. Glass structural defects cause excess losses in UV wavelength and increased loss due to exposure to UV light. Since the influence of surface tension increases in proportion to the inverse of the curvature radii of air holes, higher tension is necessary as the air holes become smaller. In the prior art disclosures, in which dispersion-tuning holes is as small as or less than ⅙ to {fraction (1/10)} of wavelength, extremely high drawing tension is necessary, resulting in a low durability against UV light and a low yield of production.